This invention relates to a process for quickly and reproducibly determining the permeability of structures made of certain barrier resin compositions.
It is well known that certain resins such as, for example, polyamides, polyesters, polyvinyl alcohol, and ethylene/vinyl alcohol copolymers have low permeability to a variety of organic fluids, such as, for example, hydrocarbons and alcohols. Such resins, when used in applications where high permeability is undesirable, are often referred to as "barrier resins". In practical use, barrier resins are combined with resins having higher permeability to organic fluids but greater strength, especially greater impact strength, which in such applications are normally referred to as structural resins. Structures containing both a barrier resin and a structural resin are available in different forms. One such form is a laminar dispersion of barrier resin in a matrix of structural resin. Another is a composite containing at least one structural resin layer and at least one barrier resin layer, and which may in addition contain adhesive resin layers or layers of other materials. Barrier resin structures are very effective low permeability materials suitable for many uses, especially for containers such as bottles, paint cans, and gasoline tanks for automobiles. This last named use is growing very fast because substitution of plastic materials for metals effects weight savings, while at the same time eliminating gasoline tank corrosion. Additional advantages of gasoline tanks made of plastic materials are better utilization of available space, since such tanks can be molded in the exact desired shape, as well as the obvious superiority of blow molding plastic resins over bending and welding metal sheets. Many countries, including the United States, have approved in principle the use of synthetic materials in manufacturing gasoline tanks.
A container which is to be used for storage or transportation of liquids evolving potentially hazardous or environmentally undesirable vapors must be manufactured from a very low permeability material; and, in fact, certain permeability criteria have been developed and are embodied in various government and industrial standards. Permeability is affected by even subtle changes in process variables, so that, contrary to the intuitive expectations, it is impossible to accurately predict in advance the permeability of a given barrier resin structure from the barrier resin content in such a structure and the nominal process conditions. It is, therefore, necessary to be able to ascertain on a regular basis whether container production runs produce products which satisfy such standards. This normally is done by placing a given liquid in a container, closing the container, leaving it for a specified period at a specified temperature, and weighing the container with the liquid at reasonable intervals until the end of the specified period. This method of determining the container's permeability is rather tedious and slow; it is not well suited for making quick production run determinations.
It is, therefore, desirable to have available an automated, instrumental process for quickly and reliably determining container permeability, so that any departures from the standards can be readily recognized, and any necessary corrective action can be taken promptly.